Solid state imaging devices, including charge coupled devices (CCD), complementary metal oxide semiconductor (CMOS) imaging devices, and others, have been used in photo imaging applications. A solid state imaging device circuit includes a focal plane array of pixel cells or pixels as an imaging sensor, each cell including a photosensor, which may be a photogate, photoconductor, a photodiode, or other photosensor having a doped region for accumulating photo-generated charge. For CMOS imaging devices, each pixel has a charge storage region, formed on or in the substrate, which is connected to the gate of an output transistor that is part of a readout circuit. The charge storage region may be constructed as a floating diffusion region. In some CMOS imaging devices, each pixel may further include at least one electronic device such as a transistor for transferring charge from the photosensor to the storage region and one device, also typically a transistor, for resetting the storage region to a predetermined charge level prior to charge transference.
In a CMOS imaging device, the active elements of a pixel perform the necessary functions of: (1) photon to charge conversion; (2) accumulation of image charge; (3) resetting the storage region to a known state; (4) storage of charge in the storage region; (5) selection of a pixel for readout; and (6) output and amplification of a signal representing pixel charge. Photo charge may be amplified when it moves from the initial charge accumulation region to the storage region. The charge at the storage region is typically converted to a pixel output voltage by a source follower output transistor.
CMOS imaging devices of the type discussed above are generally known as discussed, for example, in U.S. Pat. Nos. 6,140,630, 6,376,868, 6,310,366, 6,326,652, 6,204,524, and 6,333,205, assigned to Micron Technology, Inc.
Ideally, the digital images created by a CMOS imaging device are exact duplications of the light image projected upon the device pixel array. That is, for a flat-field image, all of the imaging pixel signals should have the same signal value. However, various noise sources can affect individual pixel outputs and thus distort the resulting digital image. As CMOS pixel arrays increase in size to obtain higher resolution, the physical non-uniformity of the arrays becomes more prominent. One issue occurring in higher resolution imaging sensors, such as, for example, eight or more megapixel sensors, is row-wise dark level non-uniformity that increases across the pixel array as the column number increases, causing a horizontal shading across the array. For example, FIG. 1 represents imaging pixel signal values of a row n of a flat-field image and shows an exponentially increasing pixel signal value as the column number increases. The increasing pixel signal value is due to row-wise dark level non-uniformity noise and will appear as a horizontal shading across the array. This shading across the array might not be significant in lower resolution imaging sensors having fewer columns of pixels or imaging devices with lower pixel clock frequencies. However, the shading across the array is more pronounced in higher resolution imaging sensors (e.g., greater than 1750 columns) and imaging devices with high pixel clock frequency (e.g., greater than 75 MHz). Accordingly, improved row-wise dark level non-uniformity compensation methods and apparatuses are needed.